Optical DQPSK is a promising modulation format that is attracting considerable commercial attention as a result of its high receiver sensitivity, high spectral efficiency (SE), high filtering and dispersion tolerance(s). Of particular interest, DQPSK may be used in combination with amplitude modulation to achieve even higher spectral efficiencies.
In optical DQPSK transmission, data is conveyed by an optical phase difference between adjacent bits. In order to detect the data contained within a DQPSK transmission, an optical demodulator is used to convert the phase-coded signal into intensity-coded signals. Typically, such optical demodulators are constructed from a pair of optical delay interferometers (ODIs).
Unfortunately, contemporary optical demodulators so constructed are quite complex, requiring precise control of the absolute phase difference between the two arms of each of the two ODIs, and precise length matching among the multiple optical paths prior to any data recovery circuits. In addition, conventional ODIs are fiber-based or planar-waveguide-based, which are temperature sensitive and therefore require precise temperature control and stabilization, particularly when employed in high performance optical transmission systems.